The conventional techniques have necessitated methods of production to be varied with melting points of raw materials fated to form spherical particles. The metallic spherical powder, such as of solder, and the spherical particles, such as of porous silica gel, are produced by the spray method using a heat-resisting spray nozzle and controlling the atmosphere at a temperature in the range of 280° C.˜330° C. in the case of solder or at a temperature in the range of 700° C.˜900° C. in the case of alkali-rich low-softening glass of silica gel.
The spherical particles of phenol resin that are commercially available include the product of Kanebo Ltd. sold under the trademark designation of “Bellpearl S” and the product of Gun Ei Chemical Industry Co., Ltd. sold under the trademark designation of Marilyn HF-050W,” for example. They are produced by a method that adopts a high-speed rotation for emulsion-polymerizing a resole resin and an aldehyde, such as formalin, to induce polymerization of ultrafine particles. JP-A 2003-203829, however, discloses a method wherein the particles have such a large particle diameter as falls in the range of 30 μm˜500 μm and, therefore, are pulverized again to such a particle diameter of practical use as falls in the range of 3˜8 μm prior to use as a material for electronic components. JP-A HEI 11-1314 proposes a method that comprises mixing a phenol resin with a cellulose derivative and a solvent, subjecting them to mutual phase separation, causing the phenol resin to harden, and thereafter removing the solvent and the cellulose. This method is complicated in process and is not disclosed so much as to a step of effecting fine division of resin and clarified as to the question on why the formation of ultrafine particles is feasible. A method that produces ultrafine particles having such a particle diameter as not exceeding 9 μm without requiring pulverization has been neither proposed nor achieved to practice to date. Such is the true state of affairs.
According to the conventional method that finely divides such a substance bearing a melting point as simple metal, alloy, glass or an organic compound with a spray nozzle at a temperature exceeding the melting point thereof, depending on the solid state properties, the particles successfully mass-produced thereby mostly have particle diameters generally measuring several mm. The particle diameters of 100 μm˜500 μm are still on the laboratory level today. A method that mass-produces particles having a diameter of not more than 10 μm without requiring pulverization, therefore, remains yet to be developed. The hot spray nozzle method suffers the nozzle to wear and corrode severely, produces particles of a large diameter and a wide particle diameter distribution, and incurs extreme difficulty in producing particles having a particle diameter distribution within a specified range even in spite of using a sieve machine. Particularly for spherical granules measuring approximately 100˜1,000 nm in diameter, industrial pulverizers and sieve machines remain yet to be developed.
Of the conventional methods described above, the method that obtains ultrafine particles by subjecting a system additionally using a second and a third substance to high-speed rotation during the course of emulsion polymerization involves a complicated process, renders removal of an impurity difficult, and incurs difficulty in accomplishing fine division without undergoing a step of pulverization.
This invention, therefore, is aimed at overcoming the problems heretofore encountered as described above and accomplishing the following improvements in properties: 1) ability to produce spherical and scaly ultrafine particles without requiring pulverization, 2) ability to obtain spherical ultrafine particles possessing a sharp spherical particle diameter distribution without requiring a sieving step, 3) ability to obtain spherical ultrafine particles extremely approximating true circle and possessing a particle diameter of 100 nm˜50,000 nm allowing selection of a size suitable for the particular purpose of use, and 4) ability to permit commercial production at a low cost.